Diagram, Diagram of the motion of a conical pendulum by Robert Hooke Diagram shows a triangle with a curve through it and various lines through the triangle marked out. Hooke attempts to explain his hypothesis for why planets moved in curved or elliptical orbits - that the curved motion came from the attractive property of a body at the centre - through a series of experiments using a pendulum. Subject: Mathematics, Geometry, Astronomy 23 May 1666